What is a vectorscope?
A vectorscope gives information about the chrominance within the image by showing the colour value of pixels. You can use the vectorscope to analyse the hue and saturation of the overall image. Chrominance is displayed on the vectorscope as an X-Y plot of hue and saturation. The hue is determined by the degree (or angle) around the circle and the saturation by the distance from the centre of the circle.
Overlaying a colour wheel onto the vectorscope we can see that it is possible to plot all the colours of the rainbow (and many more) but that the vectorscope is really just a technical tool for showing something that is actually quite simple - colour. The complexity arises because we are showing lots of information very quickly. This is because we are showing chrominance information for every pixel within every frame of video.
To give you an idea of just how much information that is, lets do a quick calculation. With a frame resolution of 1920x1080 we get 2073600 pixels for each frame of video. If we are filming at 25fps we will potentially end up plotting 51,840,000 points per second.
Drawing colour
On the vectorscope there are target points for different colours. Red, Green and Blue make up one set points. These are the primary colours of light and they are important because we can use them to make any other colour. In between the primary colour points are another set: Cyan, Magenta and Yellow. Each of these form a single target in between two primary colours. These points are the resulting colour made by mixing the two primary colours together that lie either side.
Looking at the targets on the vectorscope above we can see that between red and blue is magenta, so we know that mixing red and blue together produces magenta. Mixing blue and green together produces cyan and mixing red and green together produces yellow. The point at the centre of the vectorscope is a target for pixels with zero colour saturation. This is where black and white pixels will be drawn.
Now that we understand where colours will be drawn lets plot some onto a vectorscope. The animation running above shows a single scanline of horizontal samples. This means we have taken a single frame of video and isolated a single line of pixels (see the article on scanlines and horizontal samples for more details). As we read the samples from that line we can plot the colour onto the vectorscope. Lets have a look at each example from the animation above.
1) Starting with solid red - this simply gets drawn as a single point in the red target.
2) Half red, half white - first we draw a point on the red target then we move to the white target in the centre. As we move to the white target we draw a trace between the two points. This is an important step as it make it much easier to see what colours are being analysed.
3) Half red, half magenta - similar to above, first we draw a point on the red target then we move to the magenta target. As we move to the magenta target we draw a trace between the two points.
4) Third red, third white, third magenta - first we draw a point on the red target then we move to the white target, drawing a trace as we go. Finally we move to the magenta target, drawing another trace, thereby forming two sides of a triangle.
5) Colour bars - first we draw a point on the yellow target, then we move to the cyan followed by the green, magenta, red and finally blue. Each time we move we draw a trace between each colour.
The important thing to remember is that we are drawing a trace between each pixel analysed. It is this trace that is used to build up the final graph shown on the vectorscope.
Drawing a frame of colour bars
So far we have looked at where colours will be drawn on the vectorscope and how a trace is drawn between colours. Finally let see how a complete frame of video is analysed and displayed. In this example we are drawing colour bars.
Starting at the top of the frame we first analyse and draw a single line of samples. This is drawn on the vectorscope as a faint trace starting at the yellow target followed by the cyan, green, magenta, red and blue. As we move down each scanline we draw another faint line on top of the last. Repeating this for every line in the frame we begin to make increase the visibility of the trace. Finally after we have drawn every line we have a bright trace displayed on the vectorscope with each target being filled.
This process happens for every frame of video so it is happening imperceptibly fast. When you see a vectorscope analysing real video you see the graph constantly changing to reflect the current frame. You will also see on the Mavis vectorscope's that the trace is not green, rather, we have drawn it to reflect the corresponding colours. For example, as the trace moves to the red target it becomes red.